Those magical days when the HVAC is off

People assume its only quality of power impacting the audio systems when in fact its just as likely or more likely ambient noise levels are being affected. Furnaces/AC add both mechanical and air movement noise in your listening room. To what extent depends on type and placement of these systems. In any case higher ambient noise levels mask or cancel out low level information on recordings to some extent, and some FREQUENCIES more than others. Some have argued I make a mountain out of a molehill with this issue, and sure if you only listen to highly compressed pop/rock at high volume levels no problem. But if you listen to less compressed, quality recordings at moderate and lower volume levels perceived resolution and transparency of your system is being negatively impacted. I'd suggest the biggest reason people state their systems sound better with late night listening sessions is due to lowering of ambient noise levels as the world settles down.

Here's some examples of noise levels in db,https://noiseawareness.org/info-center/common-noise-levels/

The lowest ambient noise levels I can achieve in listening room is low 30's on quiet nights, this living on relatively quiet street, traffic is the worst for ambient noise levels. Daytime 50's and above. I can only imagine what I'd hear with something like low 20's. I'd suggest using resources to improve one's home and listening area to achieve lower ambient noise levels is equal to or better than using those resources to purchase a new higher resolving component.

@ostemo8 I didn't mean to hijack thread from your electrical concerns to the noise issue, AC quality is an important issue, its just that its common to see threads attendant to electrical noise on AC grid and or home AC, I see very few threads on the environmental noise issue. Outside of the audio equipment itself, AC quality, listening room treatment and noise pollution (both home and external generated) are equally important for maximizing system potential.

The most irritating thing about ambient noise levels is often one has very limited control over it. Rock-on has great ideas for minimizing it, but many of us don't have option to implement all of those ideas. You can turn off your HVAC at certain times, but other times you just have to put up with it, the issue is once you've experienced those magical listening sessions when ambient noise levels were at their lowest, subsequent listening sessions with higher ambient noise levels may impact your enjoyment.

@carlsbad2 correct. Just turning off AC, what about all the other contaminating electrical devices still running in your home, all of it continuing to contaminate AC. And then think about the entire AC grid, we increasingly live in a 24 hour world, I suspect for vast majority of us, grid contamination pretty much same day or night.

The biggest variable day to night is both home generated and externally generated background noise. Also, I can understand why some dismiss this as cause so easily, many live 24/7 in  high ambient noise level environments, we subconsciously block much of it out. And think about epidemic levels of hearing loss today, likely many of us suffer from it and don't even know it. Not only certain frequencies can suffer, but low level info and micro dynamics simply unheard. Think about how many of us not really hearing full potential of our systems, between ambient noise and hearing loss, could be a lot!. people simply don't know what silence sounds like anymore. Go to an anechoic sound chamber, you'll be freaked how loud just your breathing is.